Originally, VMware promoted the new vSphere 5 licensing model as being a for customers. But VMware didn't mention that each vSphere 5 CPU-based license came with a fixed amount of virtual RAM (VRAM) entitlements. If your configuration has more VRAM allocated than is entitled for use with the CPU license of vSphere 5, you need additional licenses.

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Now, VMware has made three changes that try to address concerns around VRAM as a licensing metric and, thus, the resulting increases in cost. Users commenting on VMware's forum seem to be less irate with the new changes, but they still question the VRAM model.

How does this translate to a real-world situation? Take the example of a customer with a two-socket processor with no more than 12 cores per socket, with 256GB of RAM.

Under the original vSphere 5 licensing model, the customer needed six CPU-based vSphere Enterprise Plus licenses -- not two -- to be entitled in order to use the full 256GB of RAM on the system.

Under the revised vSphere 5 licensing model, the customer needs three CPU-based vSphere Enterprise Plus licenses -- not two, as in under the vSphere 4 license.

The first two CPU-based licenses, sufficient for the two CPUs on the system, would provide 96GB of VRAM entitlement each, for a total of 192GB. However, because the scenario includes 256GB of RAM, the customer has to buy a third vSphere CPU-based licenses to use the full 256GB of RAM.

As a result, this example customer is paying 50 percent more under the revised vSphere 5 license than with vSphere 4 -- but at least its not 300 percent more, as was the case with the original vSphere 5 licensing model.

The second licensing change VMware made is that the amount of VRAM counted against any one virtual machine is now capped at 96GB. Using the example above, if your system has 256GB of physical RAM and you want to allocate 128GB of vRAM to two virtual machines, VMware reduces your vRAM allocation from 256GB to 192GB. As a result, you'd be 64GB under your 256GB VRAM entitlement even while using a full 256GB of VRAM across the two virtual machines.

In explaining the change, VMware said customers could use a 1TB VRAM virtual machine while reducing their vRAM allotment by only 96GB from the total VRAM pool. Customers would still need sufficient vSphere CPU-based licenses to use the 1TB (or however much) of physical RAM available on the system. In other words, this change applies only to how much VRAM is used from within the VRAM pool. Thus, you may need additional vSphere CPU-based licenses if you exceed the total pooled VRAM allotment later on.

Change No. 3: Measuring average VRAM usage, not maximum usage

VMware now calculates a 12-month average of consumed VRAM rather than tracking the high-water mark of VRAM used. Thus, it's less likely that you will use more than your VRAM allotment per year.

The combination of the second and third licensing change make it much more attractive to run multiple Tier 1 applications on VMware compared to the original vSphere 5 license.

Despite the changes, your costs go up, so don't put all your eggs in the VMware basketBut the cost still goes up for many customers. And risk remains that it will continue to go up: Although VMware backed off somewhat from its original fee structure, the new licensing may change in the future, after customers are heavily reliant on VMware for even their Tier 1 applications.

So it's good to evaluate options, especially as open source hypervisors become more mature. And as you move more business-critical applications to VMware, consider moving some of your existing, less-critical VMware applications and environments to an open source hypervisor. Using a mixture of VMware and an open source hypervisor is likely your best long-term option for balancing costs and flexibility.

I should state: "The postings on this site are my own and don't necessarily represent IBM's positions, strategies, or opinions."